Add stubs for json libs

[donBarbos]

cysimdjson PyPI | Github | PyPI Stats

I found this library in benchmarks article.
I didn't find any information about whether it's worth adding stubs for library depending on its popularity so I decided to open issue.
Library hasn't been updated for a long time, so there is no need to worry about support stubs.

EDIT: I also opened issue for jsonpickle PyPI | Github | Docs | PyPI Stats
jsonpickle/jsonpickle#554

If no one minds I'll be glad to send PRs

[srittau]

While we're a bit wary about adding stubs for non-maintained libraries, we have done so in the past if they were fairly stable and did work with newer Python version. But in general we welcome any new library stubs! (Of course, contributing them directly to the upstream library is even better if they accept them.)

(I'm going to close this issue for maintenance reasons, not because we don't welcome these stubs.)